1. Field of the Invention
The present invention relates to a nano-photocatalytic sol composed of spherical nano-photocatalyst and non-spherical nano-photocatalyst. A photocatalytic layer with high adhesion and good activity could be made with this nano-photocatalytic sol.
2. Description of the Related Art
Nanotechnology is the technology for producing material in the size of 10−9 meter (1 nanometer is equal to 10−9 meter), measuring its properties, and applying the special properties of such nano-sized material to the making of devices. Nanomaterials come in a wide variety and cover the fields of semiconductor, metal, polymer, biomedicine, carbon tube, etc. Nanomaterials are typically applied with their electrical, optical, magnetic, and chemical properties. The novel characteristics of nanomaterials are also applicable to industrial catalyst to enhance the surface area of the catalyst. The doping of nanomaterial can also be used to enhance the mechanical strength of devices. Turning semiconductor materials into nanosize can create high quantum confinement of electron and hole to increase the illumination efficiency and breakdown temperature of semiconductor laser. The availability of nanosized semiconductor can further reduce the size of optical and electrical components. Nanotechnology will make the integration of electronic, optical, magnetic and bio components possible.
Nano-photocatalyst have been used extensively to improve our living environment and gradually accepted by the public. Nano-photocatalyst generally means particle size under 30 nm. Under ultraviolet light irradiation (wavelength under 380 nm), active species are produced on the surface of nanoparticle which can oxidize or reduce the pollutants. In addition, the nano-photocatalyst coating layer is highly photo-induced hydrophilic, it can be applied to anti-fog, anti-dust and other self-cleaning functions. Nano-photocatalyst has been used extensively for pollutant removal, air cleansing, water purification, odor removal, anti-bacteria, anti-dust and anti-fog purposes.
Despite of their activities of anti-bacteria and pollutant removal effects, nano-photocatalysts in the form of particles cannot be used directly. The nanoparticles must be immobilized on the surface of certain substrates, e.g. ceramic, glass, wall, metal or some plastic materials. And the surface of substrate will not be oxidized or decomposed by the nano-photocatalyst. The adhesion between the nano-photocatalytic particles and substrate after immobilization is the primary factor determining the service life of photocatalyst. For convenience sake, the immobilization process is carried out with the nano-photocatalyst prepared into aqueous sol. Currently the production of nano-photocatalytic sol is produced from metal salt as starting material. In the example of common titania photocatalyst, the titanium alkanoxide salt and titanium inorganic salt are used as precursors to synthesize titania photocatalytic sol with particle size under 100 nm. Other approaches to prepare nano-photocatalytic sol including mixing nano-photocatalytic powder directly with water. However such approach needs to address further the problem of dispersion to render the nanoparticles more durable and functional in subsequent adhesion process. That is, if the nano-photocatalyst adheres strongly to the substrate, it will continue to function and becomes a product with long-standing actions on dirt removal, odor removal, anti-bacteria, anti-fog and self-cleansing.
In the development of titania nano-photocatalyst technology, ROC Patent No. 349981 discloses a method for producing non-crystalline titanium peroxide sol, in which TiCl4 is used as starting material, which is added with hydrogen peroxide under low-temperature (<15° C.) after titration with ammonia water to pH 2˜6 and then cultivated under atmospheric temperature for 7˜10 days to obtain non-crystalline titanium peroxide sol. This titanium peroxide sol addresses the coating problem. But it must be subjected further to high-temperature calcinations (250˜940° C.) to become ready as nano-photocatalytic coating, and the substrate surface must undergo hydrophilic pretreatment with surfactant to enhance the adhesion of nano-photocatalytic sol. ROC Patent No. 460321 proposes an amorphous titanium peroxide sol, which becomes crystalline titanium dioxide sol mixture after thermal treatment of 100° C. before coating the substrate, or the titanium dioxide powder is mixed with photocatalytic sol before coating the substrate. Another ROC Patent No. 491883 proposes the use of amorphous titanium peroxide sol as coating agent and ceramic containing dielectric or electric conductor as intermediate layer for coating the substrate. In ROC Patent No. 448219, surfactant and solvent or silicon dioxide are used as undercoating for coating onto substrate surface. ROC Patent No. 279175 proposes the mixture of non-degradable binder (fluoropolymer) and titania particles to form photocatalyst coating composition.
In view of prior art, some add organic additive directly into the photocatalytic sol to improve the adhesion of nano-photocatalyst to the substrate. But there exist the problems of non-uniform mixture and chemical/physical incompatibility between the additive and nano-photocatalyst particles, which will affect the stability and functionality of nano-photocatalytic sol. Some coat a layer of adhesive agent to the substrate surface before the coating of nano-photocatalyst sol to increase its adhesion to the substrate. But such approach involves more steps and limits the use of substrate material. In fact, currently available nano-photocatalytic sols tend to run into the adhesion problem where its coating layer on the substrate is prone to peel off, thereby adversely affecting the service life of photocatalyst.